Juglone paranoia! Please don't kill my tomatoes, black walnut trees!

February 6, 2009 9:10 AM

I have 2 black walnut trees in my yard. I am starting a vegetable garden this year. How should I proceed to maximize my chances for success considering the toxicity of the trees?

The trees are in the south, lower, shady side of the yard. The small raised-bed garden will be placed up on the north side, far out of their shade, I would guess maybe 20 yards away from the nearest tree. I haven't been able to find any information about how far the roots of the trees (which also produce juglone) reach.

I'm totally paranoid about compost, and at this point am not going to add any leaves at all from our yard to our compost. I did put some yard leaves into my small vermicompost bin in November without thinking -- now I'm not sure if I should even use that compost in the soil mix in the raised bed. (This makes me sad, but I don't want to poison my garden!)

I grabbed a bag of leaves from a yard in a different neighborhood, but now it's sitting in my garage because I'm paranoid that that bag might be contaminated with black walnut leaves. Wikipedia claims "About 65% of the annual wild harvest [of black walnut trees] comes from the U.S. state of Missouri" -- oh no! I may just avoid all leaves for compost, but I wonder -- will hot composting the leaves eliminate the juglone? Will vermicomposting?

What level of exposure to juglone will harm (some of) my vegetables? We definitely want to grow tomatoes and peppers and are considering doing some in pots and some in the raised-bed as a comparison, although I'd prefer not to have to grow them in pots.

posted by palegirl to home & garden (9 comments total)

This link should answer some of your questions.
posted by bolognius maximus at 9:34 AM on February 6

In reviewing that link, and noting that you have a raised bed garden, I would say you're not going to have trouble -- as long as your raised beds are deep enough that there's not root penetration into the ground. I built my raised beds from cinderblock, and three layers of cinderblock are about a foot and a half above ground, maybe 2 foot. That would certainly be sufficient for most crops. Also, import the soil from a place that isn't in a black walnut zone. (I got my soil from a dredged pond bottom, and amended it with well-aged manure.)

Hot composting doesn't seem to do much to eliminate juglone, so learn to ID black walnut and don't compost THOSE leaves. Don't know about vermicomposting, but wonder if maybe the juglone would not be good for the wormies.
posted by lleachie at 9:49 AM on February 6

The worms are definitely thriving, so that is not a concern.
posted by palegirl at 9:59 AM on February 6

I think you're being overly concerned about this.

Keep doing what you're doing. Make 2 compost piles, one for walnut leaves, the other for everything else. Keep the leaves away from the garden as much as possible. Try one or two tomato plants in a container for the heck of it the first year. The raised beds sounds like a good idea.

Have fun, and relax. I don't think you're doing subsistence farming. It'll be fine.
posted by jeff-o-matic at 10:21 AM on February 6

Research at Ohio State suggested composting breaks down the toxins. I'd guess shredding, if you have or can get the capacity, would increase this. You might also segregate a "leaf compost" bin and let it go longer (you might also want to physically segregate it from the garden so rainwater isn't leaching anything directly into the soil while the leaves are still breaking down).

They also say the root zone is 60-80 feet, so you're kind of in the gray area.

No need to wonder; sprout some seeds of susceptible plants and plant a few in the suspect compost and a few in regular potting soil and see how they do.
posted by nanojath at 10:50 AM on February 6

There are steps you can take to reduce the allelopathic effect of juglone. Clean up fallen leaves, nuts, and other debris from black walnut trees regularly to minimize their accumulation in the soil. Incorporate compost, rotted manure, composted grass clippings, and/or cover crops to maintain a high level of organic matter in the soil. This encourages a healthy population of soil microbes that can help break juglone down and minimize damage to sensitive plants. Avoid using compost that contains black walnut debris unless it has been broken down thoroughly. If you compost actively - that is, you turn the pile frequently and keep it moist - the compost can be used in about eight months. If you use the lazy method of composting - that is, pile the materials up and let them sit - you should wait at least a year before using it. You can test the safety of the finished compost by starting tomato seedlings in it. If they do not die, the mulch is safe to use.

via Penn State Cooperative Extension, 2007


My tomatoes have done really well on my patio in a container garden- another option for you.
posted by hellboundforcheddar at 11:40 AM on February 6

Just as a point of comparison - I got the best tomatoes EVER, grown in Miracle-Gro compost, in a big window box on the porch. Twice the yield of soil-grown plants. It was still economic (over store-bought) and the flavor was wonderful! My grandmother used to swear by containers for potatoes, as they were easier to harvest (she used half-barrels). You could consider the unexpected benefits of containers for susceptible plants.
posted by Susurration at 1:58 PM on February 6

Are your raised beds filled with soil from under the trees? If not, I'm not sure how the juglone would get in there, unless the beds are under the dripline of the tree. Juglone is eventually inactivated in soil, as it breaks down fairly easily. I think you'll be fine.
posted by oneirodynia at 2:51 PM on February 6

Pick the nuts up and dispose of them. I live in a neighborhood with quite a few black walnut trees (though I haven't noticed the inhibitory effect) and the rotting green tennis balls litter the sidewalks from late summer to October. It isn't worth trying to extract the nuts -- black walnuts are very hard and the husks stain.
posted by bad grammar at 5:24 PM on February 6